Surgical procedures in which a cardiovascular prosthesis is implanted into a patient's blood vessel are common in treating many vascular disorders. For example, one common type of cardiovascular prosthesis is an endovascular prosthesis that is used to strengthen a blood vessel wall in the location of an aneurysm, or to open an occlusion in a blood vessel.
A typical endovascular prosthesis includes a flexible, tubular member, made of fabric or PTFE, that may be anchored with sutures or carried by one or more support structures known as stents. Generally, each stent is formed from a material having an elasticity that is sufficient to permit radial expansion of the stent and having a strength sufficient to prevent radial collapse or burst. Such stents are typically formed from stainless steel, titanium, Nitinol, or a suitable plastic.
A common endeavor in the field of cardiovascular prosthetics is to increase the patency rate of prostheses. Thrombosis and platelet deposition on surfaces of a cardiovascular prosthesis reduce the patency rate of the prosthesis. For example, thrombosis and platelet deposition within an endovascular prosthesis may occlude the conduit defined by the endovascular prosthesis.
Many factors contribute to thrombosis and platelet deposition on the surfaces of known cardiovascular prosthesis. The most common factors are dependent upon the material or materials forming the inner surface of the conduit of the endovascular prosthesis. Typically, thrombosis and platelet deposition begin to occlude the conduit of the endovascular prosthesis when the material or materials forming the conduit of the endovascular prosthesis are foreign to the patient's body. Thrombus begins to form on the inner surface of the conduit of the endovascular prosthesis and extends annularly about the inner surface of the conduit. Eventually, the thrombus can severely restrict blood flow through the conduit defined by the endovascular prosthesis and, if left untreated, can completely occlude the conduit.
Additionally, thrombosis and platelet deposition may occur as a result of irregularities on the inner surface of a cardiovascular prosthesis. The irregularities may be formed by the structure of an inner stent that is used to support the cardiovascular prosthesis, or may be formed by the inner surface of the flexible member used for the prosthesis.
Another common type of cardiovascular prosthesis is a prosthetic cardiac valve. Prosthetic cardiac valves have been used to replace all four of the native cardiac valves. Cardiac valve replacement has traditionally been done though an invasive open surgical procedure, although percutaneous approaches are being developed.
The four native cardiac valves (mitral, aortic, tricuspid, and pulmonary) serve to direct the flow of blood through the two sides of the heart in a forward direction. On the left (systemic) side of the heart, the mitral valve is located between the left atrium and the left ventricle, while the aortic valve is located between the left ventricle and the aorta. These two valves direct oxygenated blood coming from the lungs, through the left side of the heart, into the aorta for distribution to the body. On the right (pulmonary) side of the heart, the tricuspid valve is located between the right atrium and the right ventricle, while the pulmonary valve is located between the right ventricle and the pulmonary artery. These two valves direct de-oxygenated blood coming from the body, through the right side of the heart, into the pulmonary artery for distribution to the lungs, where it again becomes re-oxygenated to begin the circuit anew.
All four of these native heart valves are passive structures that do not themselves expend any energy and do not perform any active contractile function. The valves consist of moveable leaflets that open and close in response to differential pressures on either side of the valve. The mitral and tricuspid valves are referred to as atrioventricular valves because they are situated between an atrium and a ventricle on each side of the heart. The mitral valve has two leaflets and the tricuspid valve has three leaflets. The aortic and pulmonary valves are referred to as semilunar valves because of the unique appearance of their leaflets, which are often termed “cusps” and which are shaped somewhat like a half-moon. The aortic and pulmonary valves each have three cusps.
Heart valves can exhibit abnormal anatomy and function as a result of congenital or acquired valve disease. Congenital valve abnormalities may be so severe that emergency surgery is required within the first few hours of life, or they may be well-tolerated for many years only to develop a life-threatening problem in an elderly patient. Acquired valve disease may result from causes such as rheumatic fever, degenerative disorders of the valve tissue, bacterial or fungal infections, and trauma.
The two major problems that can develop with heart valves are stenosis, in which a valve does not open properly, and insufficiency (also called regurgitation), in which a valve does not close properly. Stenosis and insufficiency may occur concomitantly in the same valve or in different valves. Both of these abnormalities increase the workload and stress placed on the heart. The severity of this increased stress on the heart, and the heart's ability to adapt to it, determine whether the abnormal valve will have to be surgically repaired or replaced.
In addition to stenosis and insufficiency of heart valves, surgery may also be required for certain types of bacterial or fungal infections in which the valve may continue to function normally, but nevertheless harbors an overgrowth of bacteria on the leaflets of the valve that may flake off (or embolize) and lodge downstream in a vital artery. If this occurs on the valves of the left side (i.e., the systemic circulation side) of the heart, embolization results in sudden loss of the blood supply to the affected body organ and immediate malfunction of that organ. The organ most commonly affected by such embolization is the brain, in which case the patient suffers a stroke. Thus, surgical replacement of either the mitral or the aortic valve may be necessary for this problem even though neither stenosis nor insufficiency of either valve is present.
If a heart valve must be replaced, there are currently several options available, and the choice of a particular type of prosthesis (i.e., artificial valve) depends on factors such as the location of the valve, the age and other specifics of the patient, and the surgeon's experiences and preferences. Available prostheses include mechanical valves, tissue valves, and homograft valves. Mechanical valves include caged-ball valves, bi-leaflet valves, and tilting disk valves. The main advantage of mechanical valves is their long-term durability. Their main disadvantage is that they require the patient to take systemic anticoagulation drugs for the rest of his or her life, because of the propensity of mechanical valves to cause blood clots to form on them. Mechanical valves can be used to replace any of the heart's four valves and are typically attached to a fabric sewing ring so that the valve prosthesis can be sutured to the patient's native tissue to hold the artificial valve in place postoperatively.
Tissue valves are typically constructed either by sewing the leaflets of porcine aortic valves to a stent (to hold the leaflets in proper position), or by constructing valve leaflets from porcine or bovine pericardial tissue and sewing them to a stent. The stents may be rigid or slightly flexible and are typically covered with a fabric, such as the material sold under the trademark DACRON™, and then attached to a sewing ring for fixation to the patient's native valve annulus. The porcine or bovine tissue is chemically treated to alleviate any antigenicity (i.e., to reduce the risk that the patient's body will reject the foreign tissue). Tissue valves may be used to replace any of the heart's four valves. The main advantage of tissue valves is that they do not cause blood clots to form as readily as do the mechanical valves, and therefore, they do not absolutely require systemic anticoagulation. Nevertheless, many surgeons do anticoagulate patients who have any type of artificial mitral valve, including tissue valves. The major disadvantage of tissue valves is that they lack the long-term durability of mechanical valves.
It should be noted that the structure associated with mechanical valves and tissue valves decreases their hemodynamic performance. Such obstructions also interfere with the normal flow patterns within and around the prosthetic valve and therefore, promote thrombosis as all artificial surfaces are thrombogenic (clot-promoting) to a greater or lesser degree.
Homograft valves are harvested from human cadavers. Homograft valves are most commonly implanted in the aortic position, but are also occasionally implanted in the pulmonary position. Homograft valves are specially prepared and frozen in liquid nitrogen, where they are stored for later use. The advantage of aortic homograft valves is that they appear to be as durable as mechanical valves, but do not promote blood clot formation and therefore do not require anticoagulation. The main disadvantage of these valves is that they are not available in sufficient numbers to satisfy the needs of patients who need new aortic or pulmonary valves. Homograft valves are also extremely expensive and can be more difficult to implant than either mechanical valves or tissue valves.